The present invention relates to a fuel supply and control system for gas turbine engines and, more particularly, to a system and method for controlling the flow of fuel to a gas turbine engine even under various postulated fuel supply and control system faults.
Typical gas turbine engine fuel supply systems include a fuel source, such as a fuel tank, and one or more pumps that take a suction on the tank and deliver pressurized fuel to the fuel manifolds in the engine combustor via a main supply line. The main supply line may include one or more valves in flow series between the pumps and the fuel manifolds. These valves generally include at least a main metering valve and a pressurizing-and-shutoff valve downstream of the main metering valve. In addition to the main supply line, many fuel supply systems also include a bypass flow line connected upstream of the metering valve that bypasses a portion of the fuel flowing in the main supply line back to the inlet of the one or more pumps, via a bypass valve. The position of the bypass valve is controlled to maintain a substantially fixed differential pressure across the main metering valve.
A redundant channel engine control system controls the operation of the engine and the fuel supply system. In particular, each of the redundant channels in the engine control system receives input parameters from the engine and aircraft and a thrust setting from the pilot. In response to these inputs, the engine control system modulates the position of the main metering valve to control the fuel flow rate to the engine fuel manifolds to maintain the desired thrust.
Fuel supply and control systems, such as the one described above, may experience certain postulated failure modes. For example, a postulated failure in the engine control system or in the fuel supply system may cause significantly higher fuel flow than commanded to one of the engines. This higher fuel flow can cause an asymmetric overthrust condition, which in some instances may lead to an overspeed shutdown of the engine. Failures that may lead to an asymmetric overthrust condition include a failure in the engine control system that causes the main metering valve to become fully-opened or to stick in a fully-opened or intermediate position, or the main metering valve may itself fail in a fully-opened or intermediate position. A sustained asymmetric overthrust condition while the aircraft is on the ground can, in some systems, cause the aircraft to exit the runway. A sustained asymmetric overthrust condition while the aircraft is in the air and on final approach to the runway can, in other systems, cause an in-flight shutdown of the engine.
Presently, fuel control systems like that described above may accommodate the postulated asymmetric overthrust conditions by including a mechanical overspeed governor (OSG), an automatic shut-off (or significant reduction) of fuel flow via an electric overspeed shutdown (OSSD), or both. Each of these features, however, presents it own disadvantages for the postulated asymmetric overthrust condition. For example, a mechanical OSG can only control fuel flow to a single, fixed, maximum setpoint. Thus, operation on a mechanical OSG alone may still lead to a sustained overthrust condition. With an electric OSSD, the pilot may not be able to control the aircraft if the postulated overthrust condition occurs. In particular, if the asymmetric overthrust condition occurred during approach to the runway, the excursion could result in the engine going from a low thrust condition, to an overthrust condition, and then to a shutdown condition.
Hence, there is a need for a system and method for controlling the supply of fuel to a gas turbine engine even under various postulated fuel supply and control system faults that overcomes one or more of the above-noted drawbacks. Namely, a fuel control system and method that, in the event of a failure that leads to an asymmetric overthrust condition, allows fuel flow control beyond a single, fixed, maximum setpoint, and/or does not result in a potentially uncontrollable engine excursion.
The present invention provides a system and method for controlling the supply of fuel to a gas turbine engine even under various system and/or component faults that may cause an asymmetric overthrust condition of the engine.
In one aspect of the present invention, and by way of example, a method of controlling fuel flow to a gas turbine engine combustor includes supplying fuel from the fuel source to a supply line. A first fraction of the fuel from the supply line is directed through a metering valve having a first variable area flow orifice to the combustor. A second fraction of the fuel from the supply line is directed through a bypass valve having a second variable area flow orifice to the fuel source. A determination is made as to whether the metering valve is controllable or non-controllable. Fuel flow to the combustor is controlled by (i) adjusting the areas of both the first and second variable area flow orifices when the metering valve is determined to be controllable and (ii) adjusting the area of only the second variable area flow orifice when the metering valve is determined to be non-controllable.
In another exemplary aspect of the invention, a system for delivering fuel from a fuel source to a gas turbine engine combustor includes a fuel supply line, a metering valve, a bypass flow line, a bypass valve, and a controller. The fuel supply line is coupled between the fuel source and the combustor for supplying fuel to the combustor. The metering valve is positioned in flow-series in the supply line. The bypass flow line is coupled between an inlet of the metering valve and the fuel source for bypassing a portion of the fuel in the fuel supply line back to the fuel source. The bypass valve is positioned in flow-series in the bypass flow line. The controller is operable to selectively adjust one of the metering valve and the bypass valve to control fuel flow rate from the fuel source to the combustor.
In yet another exemplary aspect of the invention, a controller for controlling the flow rate of fuel from a pressurized fuel source to a gas turbine engine combustor via a metering valve, includes a first valve driver circuit, a second valve driver circuit, and a processor. The first valve driver circuit is operable to generate a first valve driver signal, and the second valve driver circuit is operable to generate a second valve driver signal. The processor is operable to determine the controllability of the metering valve and, based on the determination, enable one of the first and the second valve driver circuits and disable the other.
Other independent features and advantages of the preferred sensor will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.